Protecting device against interfering electromagnetic radiation comprising EMI gaskets

ABSTRACT

The object of the invention is a device cabinet and particularly a solution for its EMI sealing (electromagnetic interference). The device cabinet comprises a first part ( 20 ) and a second part ( 21 ), which comprise a first joint surface ( 25 ) and a second joint surface ( 22 ) respectively, in a space between the joint surfaces there being an EMI gasket ( 24 ) made from a conductive wire for getting the joint surfaces ( 22, 25 ) into contact and for preventing electromagnetic interference from penetrating the joining point between the first and second parts ( 20, 21 ). It is characteristic of the invention that said EMI gasket ( 24 ) is arranged to cause friction against the joint surfaces ( 22, 25 ) for preventing and eliminating contact failures caused by the oxidisation of the joint surfaces ( 22, 25 ).

FIELD OF THE INVENTION PRIORITY CLAIM

This is a national stage of PCT application No. PCT/F100/00567, filed onJun. 22, 2000. Priority is claimed on that application, and on patentapplication Ser. No. 991452 filed in Finland on Jun. 24, 1999.

BACKGROUND OF THE INVENTION

Electromagnetic radiation generated in a device may disturb either thedevice's own operation or the operation of some external device.Generally, the aim is to protect sensitive devices and interferingsources of electromagnetic radiation against radiation by encasing themin packages made from a conductive material and by sealing the packagesso tight that no interfering electromagnetic radiation can penetrate thepackage. This type of or similar protection of devices againstelectromagnetic radiation is called EMI (electromagnetic interference)shielding.

One problematic area in EMI shielding is the sealing of junctions andjoint surfaces comprised by devices, device cabinets and boxes. If thejoint surfaces are not properly sealed with EMI gaskets, interferingelectromagnetic radiation will quite easily pass through the joint. Thebest protection against interference is achieved when the joint surfacesare tightly sealed together galvanically. This means that resistancebetween the joint surfaces, so-called ‘junction resistance’, is as lowas possible. However, it is difficult and expensive to manufacture suchplane-like joint surfaces, where the surfaces are tightly attached toeach other in every place galvanically. Therefore, solutions in which agood contact between the joint surfaces is not formed in every place butat certain distances along the whole length of the joint, are used forsealing joint surfaces. When the distance between the contacts formed issufficiently short, electromagnetic radiation can no longer penetratethe joint in disturbing quantities. A sufficient contact distancedepends on the frequency of the interfering radiation and the requiredattenuation level. Mechanical properties and the available space alsoaffect the contact distance used. In connection with device cabinets andracks, a typical contact distance can be, e.g. 5-15 mm.

EMI sealing is required in various types of electric devices. Amongothers, EMI gaskets are used in device box and cabinet doors andapertures, as well as in partitions between different units insidedevice cabinets.

There are at least three types of gaskets that are most commonly usedfor EMI sealing. In one solution, a mantle is knitted from a conductivematerial around a resilient rubber compound or some other correspondingmaterial. The mantle is knitted from a very thin wire that acts as aconductive fabric. When placed in between joint surfaces, these types ofgaskets give an even contact but do not necessarily give a sufficientcontact for EMI shielding due to the large contact area. They do notpierce through the surface, which is slightly oxidised or greasy. Thesetypes of gaskets may shed short pieces of wire, which can cause a shortcircuit after being passed on to a printed board. Neither do they endurefriction and continuous wear.

In a second solution conductive particles are mixed inside a rubber-likesealing compound, the conductive particles forming a galvanic connectionbetween joint surfaces when the joint surfaces are pressed together.However, the electroconductivity of these types of gaskets does not comenear to that of, e.g. copper alloyed gaskets. Furthermore, theproperties of these types of gaskets may change as they age.

A third solution is provided by spring-like gaskets bent from sheetmetal. Their electroconductivity is good, but their manufacture isproblematic. The manufacture of spring-like sheet metal gaskets requiresexpensive perforating and bending tools. In addition, the edges of thegaskets are sharp, whereupon one may hurt one's hand on them, and thelength of the gaskets is limited to the length of the sheet used intheir manufacture, which normally is about 70 cm, in which case afull-length gasket must be assembled from several pieces.

The most significant disadvantage of a spring-like sheet metal gasketis, however, its susceptibility to being damaged due to its poor elasticproperties. The gasket has extremely accurate tolerance of compression.If joint surfaces are pressed together too little, the gasket placed inbetween them will leak, as it is called, i.e. let electromagneticradiation significantly through it. If again joint surfaces are pressedtoo much, a permanent deformation will take place in the gasket and itscompression force will no longer be sufficient. Also in this case, thejoint will begin to leak.

FIG. 1 illustrates an EMI gasket, presented in the Patent PublicationU.S. Pat. No. 5,091,606, which comprises a helical spring 10 made fromcircular profiled wire and it may comprise a layer 11 made from aconductive and ductile material and placed on top of the spring. Whenthis type of gasket is placed between the surfaces to be sealed and thesurfaces are pressed against each other, a contact is formed between thesurfaces. The gasket is intended for sealing shafts and other surfaceswith a round cross-sectional surface, and their surrounding areas. Adisadvantage of these types of gaskets is a reasonably complexmanufacturing process, as well as the difficulty of fitting the gasketinto small spaces.

Typically, the surfaces to be sealed are made from oxidable materials,such as sheet metal, in which case when the surfaces oxidise the contactbetween the surfaces becomes weaker and EMI tightness will be lost. Thecontact also becomes weaker when dirt gets between the gasket and thesurfaces to be sealed.

SUMMARY OF THE INVENTION

Now, EMI sealing has been invented with the help of which thedisadvantages presented above can be mitigated. This is achieved withthe help of a device, which comprises a first part and a second part,joined together, which comprise a first joint surface and a second jointsurface respectively, and in between the joint surfaces an EMI gasketmade from conductive wire for getting the joint surfaces into contactand for preventing electromagnetic interference from penetrating thejoining point between the first and second parts, the device beingcharacterised in that said EMI gasket is arranged to cause frictionagainst the joint surfaces for preventing and eliminating contactfailures.

According to the invention, an EMI gasket is made from a spring-like,electroconductive wire by bending this in the appropriate shapedepending on the use, e.g. in the shape of a helical spring. The EMIgasket according to the invention is in direct contact with the surfacesto be sealed. The gasket forms reliable contacts between the surfaces tobe sealed and directs friction on to the surfaces when the device isopened and closed removing by its friction the oxide layer andimpurities possibly produced on the contact surfaces. This being thecase, the contact between the surfaces to be sealed remains good andelectromagnetic radiation cannot penetrate the joint in disturbingquantities, whereupon EMI tightness will be maintained.

Other objects and features of the present invention will become apparentfrom the following detailed description considered in conjunction withthe accompanying drawings. It is to be understood, however, that thedrawings are intended solely for purposes of illustration and not as adefinition of the limits of the invention, for which reference should bemade to the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be explained in detail by referringto the enclosed drawings, in which

FIG. 1 shows an EMI gasket according to prior art;

FIGS. 2a-2 b show one way of EMI sealing according to the invention; and

FIGS. 3a-3 d show another way of EMI sealing according to the invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

FIG. 1 was described above in connection with the description of priorart. FIGS. 2a and 2 b are related to a first embodiment according to theinvention, in which the door of a device rack is sealed with EMI gasketsaccording to the invention. The method is also suitable for use inconnection with device boxes and cabinets. Said device rack can be, e.g.one of the device racks of a base transceiver station of a mobilecommunication network, which comprises a door 20 and a frame part 21made, e.g. from an electroconductive material, such as sheet metal. Theframe 21 of the device rack comprises a tongue-like part 22 and the door20 in turn a hollow space 23 into which the tongue-like part 22 of theframe 21 can penetrate when the door 20 is closed. The penetration ofinterfering electromagnetic radiation from inside the device rack tooutside the device rack and from outside the rack to inside the rack isprevented by placing two EMI gaskets 24 according to the invention inthe hollow space 23 of said door 20 of the rack, and by closing thedoor.

In this context, by the door 20 is meant any sheet-like part that isintended for covering a hole comprised by the frame part 21 of somedevice rack. Normally, the gaskets 24 encircle the whole joint surface.Thus, the way presented here can be used for sealing all joints betweenthe door 20 and the frame part 21 of the device rack. However, in thefollowing, this paper will focus on explaining the sealing of only oneedge of the door.

FIG. 2a illustrates a situation, where the door 20 of the device rack isopen. Two EMI gaskets 24 are placed in the hollow space 23 of the door.The EMI gaskets 24 used can be made by bending the sealing wire indifferent shapes, e.g. in the shape of a polygon, but in this embodimentthe sealing wire is preferably bent in the shape of a helical spring.Said hollow space 23 preferably has a rectangular cross-section, wherethe length of the shorter sides is half the length of the longer sides.This being the case, when the door 20 is open, the two EMI gaskets 24placed in the hollow space 23 touch surfaces 22, 25 that surround them.Furthermore, the EMI gaskets 24 are preferably wound in the oppositedirections, whereupon they will not significantly penetrate into eachother when the door 20 is open.

The gaskets 24 can be made from several different materials, e.g. fromalloyed copper metal, stainless steel or other corresponding material.The gasket's electrical and mechanical properties, as well asmanufacturability can be affected by the selection of the sealingmaterial. The diameter of the wire used for the manufacture of thegasket 24 may vary, but it is typically approximately 0.5-3 mm. Thediameter of the gasket can be, e.g. 2-40 mm. The sealing wire is bent inthe required shape by a spring-making machine designed for bending wirematerial.

FIG. 2b illustrates a situation when the door 20 of the device rack isclosed. When closing the device rack door, the tongue-like part 22 ofthe frame part 21 of the rack is pressed between the EMI gaskets 24,whereupon both gaskets 24 are pressed against the surrounding surfaces22, 25 of the door 20 and the frame part 21 of the device rack. Thegaskets 24 compress in the directions that are perpendicular to the axisthat runs in the longitudinal direction inside the gasket. The elasticforces that act in the compressed gasket 24 tend to return the gasketinto its original shape, whereupon the gasket 24 presses tightly againstthe joint surfaces 22, 25 forming good contacts between the jointsurfaces 22, 25 at close distances and preventing electromagneticradiation from penetrating the joint. The distance between two adjacentcontacts, which is the same as the pitch of thread, is typicallyapproximately 10-15 mm. As a result, good EMI tightness is achieved. TheEMI tightness will not be lost even if the door 20 was slightly open, ifthe tongue-like part 22 of the device rack frame 21 still reaches as faras between the gaskets 24.

The device cabinet door 20 sealed in the way presented here is light toclose, because the gaskets 24 swing round as the tongue-like part 22that belongs to the frame 21 penetrates in between the gaskets 24.Neither does the structure demand external mechanisms for keeping thedoor 20 closed, because the forces that act in the gaskets 24 do nottend to open the door. The structure also has the advantage that, inthis case, there are more than two joint surfaces 25 (surfaces 25 onboth sides of the tongue-like part 22). This increases the EMI sealingability.

The advantages of the first embodiment according to the invention relateto friction between the gasket 24 and the surfaces 22, 25 surroundingit. The friction is particularly effective, because the helicalspring-like gasket 24 is preferably at close distances in direct contactwith the surfaces 22, 25 to be sealed. This being the case, always whenthe door 20 moves, e.g. when the door is open and closed, the threads ofthe helical spring 24 rub heavily on the joint surfaces 22, 25 keepingthe joint surfaces clean and removing the oxide layer possibly producedon them that conduct poorly electricity and weaken the contact. Thegasket 24 pressed tightly against the surfaces 22, 25 to be sealed alsoprevents the oxidisation of the surfaces to be sealed at the points ofthe contacts, whereupon the contact between the surfaces 22, 25 to besealed remains sharp.

In the first embodiment according to the invention, a gasket made byinclining the pitch of thread in the same direction along the wholelength of the helical spring-like gasket can also be used as an EMIgasket. The cross-section of the inclined helical spring-like gasket isin the shape of an ellipse so that it fits in the hollow space comprisedby the device rack door, the dimensions of which differ from thosepresented above.

In an alternative implementation of the first embodiment according tothe invention, the tongue-like part can be on the door and the hollowspace, wherein the gasket is installed, in the frame respectively.

If there is a desire to make the gasket 24 ready in the desired shape, arigid support wire placed inside the gasket can be utilised in bendingthe gasket. The support wire is first bent in the desired shape, e.g. inthe shape of the edge of the device rack, after which the gasket 24 isslipped on to the support wire. Finally, the gasket together with thesupport wire is placed in the space 23 reserved for it.

FIGS. 3a-3 c illustrate a second embodiment according to the invention.In this embodiment, the joint between a first mechanical part 30 and asecond mechanical part 31 is EMI sealed. Said mechanical parts 30, 31can be, e.g. parts of a device cabinet or rack, one of which ispreferably a door. An EMI gasket 32 is preferably bent to make aperiodic, spring-like structure so that along a specific length thespring comprises two parallel parts that are perpendicular to thelongitudinal axis of the spring and in between them a part that isparallel to the longitudinal direction of the spring. The longitudinalpart does not have to be on the same plane as the above-mentionedparallel parts, but it can be in contact with these through two curvedparts, whereupon the curved parts together with the longitudinal partform a nose-like part 34.

Preferably, said first mechanical part 30 comprises a recess 33 intowhich the EMI gasket 32 can be pushed so that it remains compressedagainst the edges of the recess 33. The nose-like part 34 of the EMIgasket reaches out of the recess 33 for forming a contact between asurface 35 of said second part 31 and the edges of the recess 33 of thefirst part 30. In the second embodiment according to the invention,longer contacts are achieved between the gasket 32 and the joint surface35 than in the first embodiment of the invention, in which a helicalspring-like gasket was used, because now the contacts are formed throughlongitudinal parts of the EMI gasket 32.

FIGS. 3b-3 c illustrate situations, where said first and second partsare separate from each other (FIG. 3b) and joined together (FIG. 3c).Friction between the gasket 32 and said second part 31 occurs alwayswhen the first part 30 moves, e.g. said first part being a door, whenthe door is opened and closed. With the friction, the same advantagesare achieved as presented in connection with the first embodimentaccording to the invention.

The nose-like part 34 of an EMI gasket 36 (FIG. 3d) can also be shapedso that two curved parts, which are parallel to each other andperpendicular to the longitudinal axis of the gasket, form a finalcontact with the joint surface 35 of said second part 31. In this case,the longitudinal part of the spring can be used to limit the motion ofthe spring against said first part 30.

This paper presents the implementation and embodiments of the inventionwith the help of examples. A person skilled in the art will appreciatethat the present invention is not restricted to details of theembodiments presented above and that the invention can also beimplemented in another form without deviating from the characteristicsof the invention. The presented embodiments should be regarded asillustrative but not restricting. Thus, the possibilities ofimplementing and using the invention are only restricted by the enclosedclaims, and the various options of implementing the invention asdetermined by the claims, including the equivalent implementations, alsobelong to the scope of the invention.

What is claimed is:
 1. An apparatus to protect the contents thereofagainst electromagnetic radiation comprising: a first part comprising arecess; a second part comprising a tongue-shaped portion, the first andsecond parts being movably connected to one another and shaped so as toform an enclosed space when the first and second parts are in a closedorientation, the tongue-shaped portion of the second part being in closeproximity to the recess of the first part when the first and secondparts are in the closed orientation; and two EMI (electromagneticinterference) gaskets, each formed from a electrically-conductive wirehaving an elongated, periodically repeating shape, the two EMI gasketsbeing mounted in the recess of the first part and being shaped so that,when the first and second parts are in the closed orientation, thetongue-shaped portion of the second part is positioned in between thetwo EMI gaskets to make electronic contact with both EMI gaskets.
 2. Theapparatus according to claim 1, wherein the first part is the door of adevice rack and the second part is the frame of the device rack.
 3. Theapparatus according to claim 2, wherein the EMI gaskets are made fromelectrically conductive wire, helical in shape.
 4. The apparatusaccording to claim 3, wherein the EMI gaskets are wound in oppositedirections.
 5. The apparatus according to claim 2, wherein the EMIgaskets make frictional contact with the tongue-shaped portion of thesecond part when the first and second parts are in the closedorientation.
 6. The apparatus according to claim 2, wherein each EMIgasket has a polygon shape perpendicular to a longitudinal axis of theEMI gasket.
 7. The apparatus according to claim 1, wherein the EMIgaskets are made from electrically conductive wire, helical in shape. 8.The apparatus according to claim 7, wherein the EMI gaskets are wound inopposite directions.
 9. The apparatus according to claim 8, wherein theEMI gaskets make frictional contact with the tongue-shaped portion ofthe second part when the first and second parts are in the closedorientation.
 10. The apparatus according to claim 7, wherein the EMIgaskets make frictional contact with the tongue-shaped portion of thesecond part when the first and second parts are in the closedorientation.
 11. The apparatus according to claim 7, wherein each EMIgasket has a polygon shape perpendicular to a longitudinal axis of theEMI gasket.
 12. The apparatus according to claim 1, wherein the EMIgaskets make frictional contact with the tongue-shaped portion of thesecond part when the first and second parts are in the closedorientation.
 13. The apparatus according to claim 1, wherein each EMIgasket has a polygon shape perpendicular to a longitudinal axis of theEMI gaskets.
 14. The apparatus according to claim 13, wherein the EMIgaskets make frictional contact with the tongue-shaped portion of thesecond part when the first and second parts are in the closedorientation.
 15. The apparatus according to claim 13, wherein the EMIgaskets are made from electrically conductive wire, helical in shape.16. The apparatus according to claim 15, wherein the EMI gaskets arewound in opposite directions.
 17. The apparatus according to claim 1,wherein each EMI, gasket comprises, along a longitudinal axis of the EMIgasket, two parallel parts that are perpendicular to the longitudinalaxis of the gasket and, in between them, a part that is parallel to thelongitudinal axis of the gasket.